In time synchronization, it is important to accurately estimate a communication delay occurring since a first terminal delivers a reference time until a second terminal receives the reference time and adjusts time. Some conventional time synchronizing devices exchange frames with counterparts a plurality of times, and select the least value from among a plurality of round-trip delay time values, to estimate the communication delay. However, when a constant delay occurs on one way communication of a round trip, the estimation accuracy of the communication delay inevitably deteriorates, and it is thus difficult to synchronize the time with high accuracy.